This invention relates to a method and apparatus for the non-destructive identification of chemicals using prompt gamma-ray neutron activation analysis. More particularly, the invention relates to non-destructive testing to identify chemicals stored in sealed containers using a portable, field deployable, isotopic, neutron bombardment, spectroscopic method and apparatus.
The invention has particular application for the identification of military chemical warfare agents located in sealed munitions. Munitions are irradiated with neutrons and the resultant prompt gamma-ray emission spectra are analyzed to reveal the chemical elements comprising the agent. The identity of the agent may then be readily determined.
The method and apparatus have further application to any situation requiring the identification of other unknown chemicals. For example, the invention could be used in airports to examine cargo or luggage. Likewise, it could be deployed to non-invasively test for hazardous or contraband materials in containerized cargo or other sealed shipping containers. The invention is also useful for identifying suspected clandestine munitions manufacturing or storage facilities.
This invention was made in the course of developing a safe, non-destructive, and portable method and apparatus for determining the contents of sealed military munitions and containers. Every year the United States military recovers a large number of munitions from the field. Most of these munitions are recovered from disposal areas, firing ranges, active and formerly active military installations, and defense munition stockpile sites.
These munitions are typically unexploded bombs, projectiles, and containers, which can be extremely dangerous. Their fuses may be armed and their casings may be fragile from corrosion and the shock of firing and landing. They may contain explosives, military obscuring smokes, practice fills, or chemical warfare agents, such as nerve agents.
The United States military identifies munition fills in bombs and projectiles by a color code, a description stenciled on each item, and lot numbers stamped in the shell casings. However, unexploded munitions may have been buried or exposed to the elements for years before they are recovered. Often, the identifying marks are obscured or obliterated by rust, abrasion, corrosion or other deterioration. The safe and lawful disposal of munitions requires that their contents be determined prior to disposal, which can be difficult when the above-listed identifying marks are not readily visible.
Currently, munitions in which the fill cannot be readily determined are initially assessed by military explosive ordinance disposal and chemical weapons experts. After initial visual examination, an expert places the munition in an airtight steel container or overpack to protect against leaks of the chemical fill. The munition is then radiographed to determine the location of internal components. After being radiographed, the munition is assessed by a neutron activation analysis technique known as portable isotopic neutron spectroscopy (sometimes referred to simply as xe2x80x9cPINSxe2x80x9d).
The United States military has used the PINS system for non-destructive identification of suspect chemical munitions and containers for several years. The contents of items analyzed by PINS have varied from muddy water and sand to chemical warfare agents including the nerve agent sarin, mustard gas, lewisite, and blood agents. The PINS system employs neutron radiation to probe the chemical elements within a sealed munition or container. The output of the PINS assay is in the form of uncalibrated, raw gamma-ray spectra from a portable high-purity germanium gamma-ray spectrometer. The process of generating the gamma-ray spectra is usually performed in the field, i.e., at a munitions disposal site. These gamma-ray spectra are then sent to a nuclear laboratory for expert interpretation to identify the contents of the container or munition.
The data produced by the portable isotopic neutron spectroscopy method has to be analyzed in the laboratory by highly skilled and experienced scientists and engineers to correctly assess whether any dangerous chemicals are present in the containers or munitions. This requirement for such specialized personnel often seriously extends the period between examination of a munition and its disposal. In addition, human judgment errors regarding the contents of a chemical warfare munition are possible and may result in improper handling or disposal of the chemical warfare munition.
Accordingly, prior to development of the present invention, there had been a long-felt need for a portable isotopic neutron spectroscopy method and apparatus that could be operated in the field by persons not having extensive scientific and technical knowledge of physics and chemistry.
A method and apparatus for quickly determining the chemical composition of a chemical agent in a container are provided. The method involves exposing the agent to neutrons that excite the atomic nuclei of the agent to produce gamma-ray emissions. A radiation detector senses the gamma-ray emissions generated by the agent and outputs data that is representative of the energy levels of the emissions. The data is typically in the form of voltage pulses that are representative of the energy of the gamma rays. The data is then processed by a multichannel analyzer, which generates a spectrum of the gamma-ray emissions. The gamma-ray spectrum is automatically energy-calibrated and analyzed to identify the chemical elements that are present in the agent. The relative amounts of the chemical elements present in the agent are also determined.
Based on the chemical elements comprising the agent, the chemical compound or mixture of the agent is assessed via a decision tree algorithm. A decision tree algorithm is executed, wherein the relative amounts of the chemical elements in the agent are compared to a look up table of chemical compositions with known relative amounts of the same chemical elements. When a match is made between the chemical elements of the agent and the table, the match is reported by a computer in an easily readable format that identifies the agent in plain language for the operator.
Analysis of the gamma-ray spectral data begins with calibrating the energy scale of the spectrum. The voltage of each pulse arriving at the multichannel analyzer is proportional to the ionization energy deposited in the sensitive volume of the detector. After amplification and reshaping, the multichannel amplifier sorts each pulse according to its voltage into a histogram bin or xe2x80x9cchannel.xe2x80x9d The number of pulses or xe2x80x9ccountsxe2x80x9d per channel represents the gamma-ray intensity at a given energy level. A complete gamma-ray spectrum typically has 4,096 to 16,384 channels and is similar to a graph of gamma-ray intensity on a y-axis versus channel and/or detector pulse-height voltage on an x-axis. The energy calibration procedure, in effect, rescales the x-axis in energy rather than voltage units, e.g., thousands of electron volts (xe2x80x9ckiloelectron voltsxe2x80x9d or xe2x80x9ckeVxe2x80x9d hereinafter). Energy calibration is necessary because the amplification factor or gain of the detector and the multichannel analyzer may change over time due to thermal or random effects.
The energy calibration of the spectroscopy system is derived from the positions of neutron-induced gamma rays or natural background radiation observed in most or all spectra recorded by the detector. Specifically, these include gamma rays produced by neutron interactions in the detector itself, neutron interactions in shielding materials surrounding the detector, and neutron interactions in the container or munition wall materials. For example, steel in a munition casing interacts with the neutrons and causes specific gamma-ray emissions, e.g., emissions representative of iron.
An energy calibration algorithm converts the x-axis scale from channels to kiloelectron volts using an expression of the form E(i)=a+bi+ci2, where i is the channel number, E(i) is the energy of the ith channel in keV, and a, b, and c are calibration constants to be determined by the algorithm.
To determine the calibration constants, the algorithm first searches the spectrum for peaks significantly above the background. The centroids of these peaks are matched to a specified pattern to identify the individual peaks. Then, a non-linear least square fit of experimental peak energies versus expected peak energies is performed to determine the three constants.
Once calibrated, the gamma-ray spectrum is direct reading. The spectrum is then searched for selected chemical elements by performing a directed peak fit of the corresponding gamma-ray energies. For those peaks detected at a significant level above the background, peak centroid energies and net peak areas are extracted from the spectrum to determine the gamma-ray counting rates for the chemical elements of interest.
A decision tree algorithm is then executed to identify the fill agent based on the chemical elements of which the fill agent is comprised. The decision tree first narrows the possible fill agent choices by the presence and absence of various chemical elements. For example, if phosphorus is detected in a munition under test, there are only two possible fills, white phosphorus smoke, or an organophosphorus nerve agent. The final step in the decision tree is to compare the gamma-ray count rates from selected chemical elements to determine the best match from the remaining possibilities. Continuing with the previous example, white phosphorus munitions typically contain 70-100 weight-% phosphorus and 0-3 weight-% hydrogen, while nerve agents normally contain 11-22 weight-% phosphorus and 7-10 weight-% hydrogen. The spectral gamma-ray counting rates faithfully reflect these ratios. Finally, the computer reports the identity of the fill agent to the system operator in an easily readable format.